(but usually won't)
Dunno if it's ever been explicitly stated in any of the various scifi settings, but I still hold the headcanon that 80s "cassette futurism" stuff is actually totally reasonable because at some point humanity decided it was better to just mass produce simpler more reliable "old" tech that still worked just fine. Like maybe the homeworld has star trek level infinitely advanced iphones, but industrial ships and outer colony worlds prefer clunky boxes than can take a beating and can actually be repaired in-flight.
"Hey, this homeworld tech uses microcircuitry and a dozen specialty materials you can't even get in this sector... but this 80lb machine based on an archived schematic from the late 1900s that does exactly one thing can be made right here at home and if you get desperate you can probably fix it with copper wire"
This is the Mikoyan-Gurevich MiG-25 "Foxbat", there's a lot of context around the construction and service life of this plane that explains why it is the way it is but for brevity's sake I'll say that a central design interest for Mikoyan-Gurevich was that this plane be as fast as an interceptor could be, at as high an altitude as possible. This was the USSR in the 60s though, building an aircraft that could scrape the stratosphere carrying guided missiles was a tall order and corners quickly started getting cut.
Optimally an aircraft like this would use a titanium airframe to survive friction and exhaust heating at high speeds without incurring too much weight, but titanium was expensive and hard to work with so Mikoyan-Gurevich favored hand-welded steel and assessed that many panels could actually be riveted in place without affecting the aerodynamics at an even lower cost.
Needing to propel such a massive aircraft to unreasonable speeds and altitudes the bureau turned to a recently canceled project for high altitude cruise missiles and borrowed two of these engines to meet the speed requirements of the craft at a low cost, longevity and efficiency be damned.
In order to mitigate the need for managing delicate and temperature-sensitive solid-state avionics the craft is run on a series of vacuum tube computers, using the latest "nuvistors" to keep the scale down this nonetheless resulted in a temperature-resistant "brain" for the aircraft with two dramatic benefits: The plane could direct a massive amount of power into its radar without worry about heat build-up, and was functionally immune to any theoretical form of electromagnetic interference (a very real concern at the height of the Cold War).
Basically in building a plane to go as fast as possible as cheaply as possible the Soviets managed to just avoid having to deal with a bunch of operational hazards and considerations through sturdy and proven technologies.
Lockheed's solution to the same problem of getting an interceptor to Mach 3 (yes, I know only 3 YF-12s were produced and this mostly applies to the more numerous A-12s and SR-71s, but as much as I love the F-108 it never got built so this is the most direct comparison to the Foxbat) is the exact opposite of the MiG-25 and I find it incredibly funny
Titanium hard to come by? (mostly because the Soviets controlled something like 90% the world supply) Set up a Rube-Goldstein-esque series of shell companies and straw buyers to import the necessary ore from your archenemy!
How do you propel the giant interceptor you've built at Mach 3 for extended periods of time? Get Pratt and Whitney to redesign the J58 (fun fact! the J58 was originally designed to power the P6M, a dang flying boat) into a turboramjet1 to allow you to cruise at max afterburner for long periods of time.
I just think there's something super funny in contrasting Mikoyan's very "eh, it works for its intended purpose, just don't let Misha push her past Mach 2.8 or the crew chief will beat him to death for ruining the engines" to Lockheed going all in on letting Kelly Johnson enter a cocaine-fueled flow state and for him to walk out of the desert several days later with the complete plans to the fastest air-breathing aircraft ever built
1. Okay, technically not a turboramjet due to the arguably academic distinction that the compressors are never fully bypassed. However, it operates on a similar principle and the J58 is often described as one.
The USA just didn't have enough of the stuff and had to procure it through a Machiavellian web of shell companies. Meanwhile the USSR basically just couldn't alloy it into something useful for aircraft because their aviation industry consisted of a network of dudes in sheds with power tools.
I don't want to shit on the soviet aerospace industry too bad, because they did create some genuinely impressive aircraft, but I will forever find it funny that there was a continuous cycle of soviet engineers building a thing that was okay at its job, soviet propagandists insisting it was a worldbeater, and then the Americans taking them at face value, panicking, and building said worldbeater they thought the soviets had in the first place
